I decided to make the baby sexing page separate, because not everyone wants to get into anatomy. At different points in development, rats can be sexed by different signs. The most obvious indicator at any age is the distance between the "bump" (in males it is the penis, in females it's the urethra) is from the anus. The distance is larger in males because they have testes. After the testes drop, sex differences become very obvious. The males' "bump" is often larger than the females, but this is not a sure indicator. When fur starts to develop, another sign appears. In rats, males have no teats, so you can sex them by their "nipple spots." (BTW, rats have belly buttons. Don't mistake it for a nipple. ;) )